The field of the invention is electric powered material handling vehicles, and more particularly lift trucks which perform lifting operations under a variety of operating conditions.
Electric powered lift trucks employ large lead acid batteries to provide energy for their traction systems and their lift systems. The battery typically delivers from 24 to 48 volts to a DC bus and a traction system is powered from this bus to move the truck around the workplace under the control of an operator or a computer. The traction system can draw large currents from the DC bus during truck acceleration or when moving the truck up an incline, and it can regenerate current into the DC bus when “plugging” during truck deceleration or movement down an incline.
The lift system typically consumes half the power during normal truck operation. When lifting heavy loads the lift system draws current from the DC bus and when lowering a load it may regenerate current back to the DC bus.
A conventional lift truck will typically operate from 5 to 6 hours on a fully charged battery. When the battery voltage drops below a certain level the truck is driven to a battery station where the depleted battery is removed and a fully charged replacement battery is installed. This operation typically requires from 20 to 30 minutes during which the truck and operator are nonproductive.
Efforts have been made to increase the efficiency of these drive systems and to thereby lengthen the time period between battery changes. This includes recharging the battery during truck operation with the currents regenerated by the traction system and the lift system. While this approach does recover some of the energy, lead acid batteries are very inefficient energy recovery devices and much of the regenerated energy is wasted as heat produced in the battery. Also, the high current peaks that can occur during truck operation can limit battery life.